The present invention relates generally to an objective lens system for endoscopes and more particularly to an objective lens system for endoscopes which has a wide field angle, is made compact because of having a short total length and a short outer diameter, and is limited in the number of lenses used.
Among objectives lenses for endoscopes known so far in the art, there is a retrofocus type of lens such as that shown in FIG. 1 (see Japanese Provisional Patent Publication No. Sho. 63-281112 for example). This retrofocus type of objective lens comprises negative and positive lens components on the object and image sides, respectively, between which a stop S is located. This objective lens enables the negative lens component located in front of the stop S to deflect a principal ray P so strongly that a wide field angle can be achieved, and further allows the positive lens component in the rear of the stop S to make the principal ray P incident on the image plane parallel with the optical axis, causing the light flux to strike vertically on an image guide G.
In recent years, however, video scopes using various solid-state image sensors instead of image guides have often been used as well. When a video scope is used, however, a picture on the monitor screen lacks precision in terms of the colors reconstructed, because the built-in solid-state image sensor is sensitive to infrared light other than visual light. For this reason, it is required to use a filter for cutting off infrared light (an infrared cutoff filter). Also, when laser light lying in the near to far infrared range is used with a video scope for medical purposes, a solid-state image sensor such as a CCD is saturated with this laser light, giving rise to a smear or blooming, which otherwise makes it difficult to view the internal part to be examined. It is thus required to locate within the optical system a filter for cutting off light having a wavelength corresponding to that of the laser light used (e.g., an YAG cutoff filter).
Referring here to an optical system of such a type as set forth in Japanese Provisional Patent Publication No. Hei. 1-217410 (see FIG. 2), however, the insertion of such filters between the first and second lens components and the second and third lens components renders it impossible for them to show their own properties, because the angle of inclination of off-axial marginal rays with respect to the optical axis is very large. In the case of an absorption type of filter, on the one hand, a large angle of incidence of the rays is responsible for a color variation on the screen, because there is a difference in the optical paths due to a difference in the image height. In the case of an interference type of infrared cutoff filter, on the other hand, an increase in the angle of incidence of the rays induces a sharp change in the spectral transmittance characteristic and a rapid increase in the infrared transmittance, making it impossible to cut off rays lying in the infrared range sufficiently.
Referring again to the optical system of the type as disclosed in Japanese Provisional Patent Publication No. Sho. 63-281112, the space between the 3rd lens component and the image plane is narrow; that is, some difficulty is encountered in the insertion of a filter in it. Enlarging the space between the 3rd lens component and the image plane to allow for the insertion of a filter is unsuited for an endoscopic objective lens, because the outer diameter of the 3rd lens component becomes too large.
In such a conventional objective lens for endoscopes as shown in FIGS. 1 or 2, the chromatic aberration of magnification is compensated for only by the cemented lens of the rear unit; that is, the chromatic aberration of magnification caused by the negative lens of the front unit cannot be well made up for. In order to lower the ray height in the 1st lens component for the purpose of achieving a wide field angle and reducing the outer diameter of the 1st lens component, it is also required to increase the negative power of the 1st lens component. With the conventional lens system shown in FIGS. 1 or 2, however, the asymmetry of a large coma caused by increasing the power of the 1st lens component cannot be compensated for by the 2nd, . . . lens components. In addition, when the lens refractive index of the 1st lens component are increased with a view to increasing the negative power of the 1st lens component, the dispersion of that lens becomes too high to make up for a large chromatic aberration taking place through the 1st lens component by the 2nd, . . . lens components. For this reason, when it is intended to impart a wide field angle to the conventional retrofocus type of objective lens for endoscopes, it is subject to a problem that the outer diameter of the first lens component becomes large.